1.Field of the Invention
The present invention relates to a pixel circuit including a light emitting element and a driving method thereof and, in particular, to a pixel circuit applicable to a display apparatus of a self-light-emitting type and a driving method thereof.
2. Description of the Related Art
An display apparatus of the self-light-emitting type, which is typified by an organic electroluminescence (EL) display apparatus, is configured by arranging a plurality of pixels in a matrix on a substrate. In each pixel, a pixel circuit including a light emitting element is arranged. To cause the light emitting element of each pixel to emit light with a luminance based on an image data of each pixel, a current rate flowing into each light emitting element is required to be accurately controlled. Typically, a display apparatus of the self-light-emitting type has an active matrix configuration where an active element (which hereinafter may be called “Tr”), such as a thin film transistor (TFT), is provided on a pixel circuit, to accurately control the current rate flowing in each light emitting element.
Incidentally, TFT formed of polycrystalline silicon (polysilicon, hereinafter called “P-Si”) has a higher field effect mobility and a higher ON current than TFT formed of amorphous silicon (hereinafter called “A-Si”). Accordingly, the transistor Tr formed of polysilicon is suitable to the transistor Tr in a pixel circuit used in a high resolution display apparatus. However, the transistor Tr formed of polysilicon has a problem in that variation in electric characteristics tends to occur owing to lattice defect on crystalline grain boundaries. To address the problem, there are techniques described in Japanese Patent Application Laid-Open Nos. 2008-176287 and 2006-251631 that correct variation in threshold of the transistor Tr (variation in threshold voltage) in a pixel circuit with a source follower type connection where the cathode electrode of a light emitting element is connected to a constant voltage and the anode electrode is connected to the source electrode of a driving transistor.
Japanese Patent Application Laid-Open No. 2008-176287 adopts one of a method of driving by changing the potential of a power source line connected to a pixel circuit and a method of driving without changing the potential of a power source line, in the case where a pixel circuit corrects variation in threshold adopting NMOS as a driving the transistor Tr.
However, in the case of driving by changing the potential of the power source line, the power source line typically has a wide wiring width to reduce the resistance and thus has a high parasitic capacitance. Accordingly, there is a problem in that change in potential of the power source line increases power consumption. At the same time, a switch for changing the potential of the power source line is arranged, which causes a problem in that it is difficult to achieve high resolution. Meanwhile, in the case of driving without changing the potential of the power source line, a voltage higher than the potential of the power source line is applied as a pre-charge voltage to the pixel circuit to thereby perform driving. Accordingly, there is a problem in that the required voltage range is increased and thus power consumption is increased.
Japanese Patent Application Laid-Open No. 2006-251631adopts a method of driving without changing the power source line, includes two capacitors in a pixel circuit for correcting variation in threshold, and writes a data voltage using a capacitance ratio. However, requirement of the plurality of capacitors in the pixel circuit causes a problem in that it is difficult to achieve high resolution.